In deep-water hydrocarbon (e.g. oil, gas or mixtures thereof) extraction, crude oil or gas is extracted from below the sea floor and transferred via a pipeline system to the surface of the water. It is critically important to maintain the temperature of the oil or gas flowing through the pipeline, which typically is extracted at elevated temperatures (e.g., 60-300° C.), at temperatures above about 40° C. to avoid the precipitation of solid materials and hydrates which can lead to plugging of the pipeline and can interfere with production. As the water temperature at great depths is slightly above freezing temperature (e.g. about 4° C.), provision must be made to insulate the pipelines.
Further, if oil or gas flow must be interrupted for well maintenance or because of inclement weather conditions affecting surface platforms and interrupting pumping operations, it is important to maintain the temperature of residual crudes and gases within the pipeline and other components of the pipeline system (e.g., Christmas trees or subsea trees, risers, and the like) above precipitation temperatures for the particular crudes or gases being extracted in order to minimize or completely avoid the expensive and production-interrupting necessity of declogging and/or flushing the pipeline system before resuming production.
To this end, many efforts have been undertaken to provide economical and efficacious solutions to the problem of insulating underwater oil and gas pipeline systems. A well-accepted method is to provide a pipeline comprising a pipe-in-pipe system wherein an inner pipe is surrounded by an outer pipe serving as a carrier pipe, and wherein the annular space defined by inner pipe and outer pipe contains an insulating material. For example, U.S. Pat. No. 6,145,547 discloses a pipe-in-pipe assembly comprising a self-sustaining plate of microporous material surrounding an inner carrier pipe and encased by an outer carrier pipe, wherein a free passageway is provided for longitudinal gas flow. The assembly is maintained at reduced pressure for improved thermal insulation.
U.S. Patent Application Publication 2004/0134556 A1 discloses a heat insulating system for tubular bodies (e.g., a pipe-in-pipe assembly) comprising at least two superimposed evacuated panels, each of which is separately placed around the inner pipe of the pipe-in-pipe assembly, and wherein the two opposed edges defining gaps of each of the at least two panels are placed so as not to coincide and thus eliminate a continuous passageway for the transfer of heat between the inner and outer pipes.
Similarly, there is great interest in pipelines for transporting liquefied hydrocarbons (e.g. liquefied natural gas, liquefied propane gas). In this case, thermal insulation is required to maintain the low temperature of the liquefied natural gas (about −163° C.) to avoid vaporization of the liquid due to heat transfer from the warmer surroundings.
Additionally, steam injection is often employed to maintain reservoir pressure in oil and gas fields as the fields become depleted and thus to maintain production at an economic rate. In such a technique, steam must be transported to the production site, which is often distant from the site of steam generation. Accordingly, thermal insulation of the steam-carrying pipes is required to prevent condensation of the steam.
The use of polyurethane foam in pipe-in-pipe systems is commonly known by those practiced in the art. Some polyurethane foam pipe-in-pipe systems adhesively bond the inner and outer pipes to allow for load transfer. While this method can be acceptable, once excessive force breaks the adhesive bond, the value of longitudinal load transfer (and, potentially, radial load transfer) is lost.
The transfer of hot fluids and cryogenic fluids (for example industrial gases such as oxygen, nitrogen, argon and hydrogen) in industrial plants, HVAC systems, steam heating systems for corporate, municipality, or university campuses and buildings) and many other environments also requires insulation. In some of these cases, the outer pipe is a simple cover comprising a material such as aluminum cladding or PVC pipe.
Many existing methods of insulating pipe-in-pipe assemblies remain deficient in numerous respects. Pre-formed insulating panels and the like, of necessity retain gaps in insulation when placed within pipe-in-pipe assemblies, both between their opposing edges and between ends when laid end-to-end, allowing for heat transfer between inner and outer pipes, which reduces insulation efficiency and requires greater amounts of insulating materials.
Maintenance of reduced pressure within the annular space of some pipe-in-pipe assemblies places great demands on forming vacuum-tight assemblies and places the performance of the assembly at risk should the vacuum be compromised. Some insulating materials such as polyurethane foam lose insulation efficiency and/or shape over service life. Other insulating materials require the use of a larger diameter outer pipe to accommodate sufficient insulating material due to less efficient insulation capabilities.